U.S. patent number 7,371,294 [Application Number 11/045,309] was granted by the patent office on 2008-05-13 for high-strength cold-rolled steel sheet having outstanding elongation and superior stretch flange formability and method for production therof.
This patent grant is currently assigned to Kobe Steel, Ltd.. Invention is credited to Shinji Kozuma, Masaaki Miura, Yoichi Mukai, Yoshinobu Omiya.
United States Patent |
7,371,294 |
Miura , et al. |
May 13, 2008 |
**Please see images for:
( Certificate of Correction ) ** |
High-strength cold-rolled steel sheet having outstanding elongation
and superior stretch flange formability and method for production
therof
Abstract
A high-strength cold-rolled steel sheet is disclosed that
comprises a steel including C: 0.05 to 0.13 mass %, Si: 0.5 to 2.5
mass %, and Mn: 0.5 to 3.5 mass %, as well as Mo: 0.05 to 0.6 mass
% and/or Cr: 0.05 to 1.0 mass %. The steel sheet is of composite
structure of a ferrite+a second phase wherein the second phase has
an area rate of 30 to 70% and is combined approximately in a shape
of a network; a circle-equivalent average ferrite grain size is not
more than 10 .mu.m; and a circle-equivalent diameter of ferrite
grain aggregate that exists continuously in an area surrounded by
the second phase is not more than 3 times of the average ferrite
grain size. The steel sheet has a high-strength and satisfies a
balance of elongation and stretch flange formability (ratio of hole
expansion) at a higher level.
Inventors: |
Miura; Masaaki (Kakogawa,
JP), Mukai; Yoichi (Kakogawa, JP), Omiya;
Yoshinobu (Kakogawa, JP), Kozuma; Shinji
(Kakogawa, JP) |
Assignee: |
Kobe Steel, Ltd. (Kobe-shi,
JP)
|
Family
ID: |
34805813 |
Appl.
No.: |
11/045,309 |
Filed: |
January 31, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050167007 A1 |
Aug 4, 2005 |
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Foreign Application Priority Data
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Feb 2, 2004 [JP] |
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2004-025777 |
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Current U.S.
Class: |
148/333; 148/320;
148/328; 148/334 |
Current CPC
Class: |
C22C
38/02 (20130101); C22C 38/22 (20130101); C22C
38/38 (20130101) |
Current International
Class: |
C22C
38/22 (20060101); C22C 38/12 (20060101) |
Field of
Search: |
;148/333-334,320,328,603,650-652 ;420/104-111,123-124 |
Foreign Patent Documents
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63-241115 |
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Oct 1988 |
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JP |
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63-293121 |
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Nov 1988 |
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JP |
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9-67645 |
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Mar 1997 |
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JP |
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362074024 |
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Apr 1997 |
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JP |
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10-237547 |
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Sep 1998 |
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JP |
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11-350038 |
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Dec 1999 |
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JP |
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Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
The invention claimed is:
1. A high-strength cold-rolled steel sheet, excellent in elongation
and stretch flange formability, which comprises a steel including
C: 0.05 to 0.13 mass %, Si: 0.5 to 2.5 mass %, Mn: 0.5 to 3.5 mass
%, and at least one of Mo: 0.05 to 0.6 mass % and Cr: 0.05 to 1.0
mass %, wherein the steel is of composite structure of a ferrite +
a second phase, the second phase having an area ratio of 30 to 70%
and being combined approximately in a shape of a network, a
circle-equivalent average ferrite grain size being not more than 10
.mu.m, and a circle-equivalent diameter of ferrite grain aggregate
that exists continuously in an area surrounded by the second phase
being not more than 3 times of the average ferrite grain size.
2. The high-strength cold-rolled steel sheet according to claim 1,
further including at least one element selected from a group
composed of Ti: 0.005 to 0.05 mass %, Nb: 0.005 to 0.05 mass %, and
V: 0.005 to 0.2 mass %.
3. The high-strength cold-rolled steel sheet according to claim 1,
wherein the second phase is mainly of tempered martensite or of
martensite.
4. The high-strength cold-rolled steel sheet according to claim 1,
wherein a ratio (HvII/Hv.alpha.) of an average hardness (HvII) of
the second phase to an average hardness (Hv.alpha.) of the ferrite
phase is not more than 3.0.
5. The high-strength cold-rolled steel sheet according to claim 1,
wherein an elongation (El) is not less than 14%, and a stretch
flange formability (.lamda.) is not less than 50%.
6. The high-strength cold-rolled steel sheet according to claim 1,
wherein a tensile strength (Ts) is not less than 780 MPa.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength cold-rolled steel
sheet having a composite structure composed of a ferrite and a
second phase (mainly martensite), excellent elongation and stretch
flange formability, and superior workability, and also relates to a
method for production thereof.
For example, steel sheets used for an automobile demands
high-strength and excellent molding workability, taking into
account both passenger's safety and body-weight reduction for
saving fuel consumption. Various strengthening methods are adopted
in manufacture of high-strength steel sheets. Especially as
high-strength steel sheets strengthened using hard martensitic
structure, much attention has been focused on a composite structure
steel sheet having a ferrite-martensite two-phase structure.
The present inventors have investigated steel sheets with a
composite structure having high-strength and superior workability.
The present inventors has already proposed high-strength
cold-rolled steel sheets having superior workability described in
Japanese Patent Laid-Open (JP-A) Nos. 63-241115, 63-293121,
9-67645, 10-237547. All of these Referential Patents secure
workability by a soft ferrite phase as a first phase (main phase)
by specifying a content of C, Si, and Mn as basic components, and
simultaneously, by using steel including proper quantity of Cr, Mo,
etc., and by controlling cold rolling conditions and cooling
conditions after hot-rolling, conditions of subsequent heat
treatment and aging treatment, etc. They also realize coexistence
of strength and workability by securing strength by precipitation
of a low temperature transformation forming phase of martensite
etc. having structure strengthening effect.
In recent years, it has become clear that adjustment of a hardness
ratio and a hardness difference between a ferrite phase and a low
temperature transformation forming phase in the steel sheet with a
composite structure can improve stretch flange formability
(.lamda.) regarded as important for forming workability. In more
detail, it has also become clear that a smaller hardness ratio and
a smaller hardness difference can further improve stretch flange
formability.
JP-A No. 11-350038 discloses a technique wherein a combination of
suitable steel compositions and manufacturing conditions give
suitable composite structure, and enables production of a
cold-rolled steel sheet having superior elongation and stretch
flange formability with concurrent secure of high-strength. The
JP-A No. 11-350038 specifies a content of Nb, Ti or V as important
additional trace elements, and it also clarifies that skillful use
of refining effect of crystal grains by fine carbide, caused by
addition of these elements, produced in steel gives both of
excellent ductility and stretch flange formability.
The steel sheet with the composite structure is excellent in
compatibility between a high-strength and excellent elongation and
stretch flange formability. However, in recent years, there have
been increased demands for thin-walled and light-weighted material
steel sheets and yet improved processing efficiency. To cope with
this, high-strength steel sheet having excellent elongation and
stretch flange formability exceeding the conventional technique
level would be needed.
SUMMARY OF THE INVENTION
Under the circumstances, the present invention aims to provide a
high-strength cold-rolled steel sheet that can attain a higher
level of balance of elongation and stretch flange formability
(ratio of hole expansion:.lamda.), while guaranteeing a strength of
780 MPa needed as a steel sheet for automobiles etc.
One aspect of the present invention resides in a high-strength
cold-rolled steel sheet that has superior elongation and superior
stretch flange formability. The high-strength cold-rolled steel
sheet comprises a steel including C: 0.05 to 0.13 mass %, Si: 0.5
to 2.5 mass %, and Mn: 0.5 to 3.5 mass %, as well as Mo: 0.05 to
0.6 mass % and/or Cr: 0.05 to 1.0 mass %. The high-strength
cold-rolled steel sheet is of composite structure of a ferrite+a
second phase (exclusive of ferrite) wherein the second phase has an
area rate of 30 to 70% and is combined approximately in a shape of
a network; a circle-equivalent average ferrite grain size is not
more than 10 .mu.m; and a circle-equivalent diameter of ferrite
grain aggregate that exists continuously in an area surrounded by
the second phase is not more than 3 times of the average ferrite
grain size.
A term of "approximately in a shape of a network" means that a case
is included where the structure may not have a perfect network. A
term of "circle-equivalent" in a circle-equivalent average ferrite
grain size and a circle-equivalent diameter used herein mean a
diameter of a circle having a same area.
In the aspect of the present invention, the high-strength
cold-rolled steel sheet may also include at least one element
selected from a group composed of Ti: 0.005 to 0.05 mass %, Nb:
0.005 to 0.05 mass %, and V: 0.005 to 0.2 mass %. These elements
have a refining effect of crystal grains and contribute to further
improvement in elongation and stretch flange formability. The
second phase constituting the metallographic structure may be of
martensite and of bainite. In order to aim at coexistence of
elongation and stretch flange formability while securing
high-strength, a more preferable second phase structure is of
martensite or of tempered martensite.
In the aspect, in order to secure superior balance of elongation
and stretch flange formability in a desired level of the present
invention, a ratio (HvII/Hv.alpha.) between an average hardness of
the second phase (HvII) and an average hardness (Hv.alpha.) of the
ferrite phase is preferably not more than 3.0.
In the aspect, the high-strength cold-rolled steel sheet of the
present invention is characterized by its superior balance of
strength and workability, that is, a hardness level of not less
than 780 MPa, not less than 14% of elongation (El), and not less
than 50% of stretch flange formability (.lamda.).
The aspect of the present invention permits a cold-rolled steel
sheet having a high-strength and satisfying a balance of elongation
and stretch flange formability (ratio of hole expansion) at a
higher level as compared to conventional materials. Use of the
cold-rolled steel sheet of the present invention especially for
automotive structural material etc. can save the vehicle body
weight, and thereby providing a very useful material focused on
reduction of fuel consumption and low-pollution vehicles.
Other and further objects, features and advantages of the invention
will appear more fully from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an SEM photograph showing a microstructure of a steel
sheet with a composite structure obtained in the Example;
FIG. 2 shows an SEM photograph showing a microstructure of a steel
sheet with a composite structure as a comparative material;
FIG. 3 is a schematical diagram conceptually showing an expanded
microstructure photograph of a steel sheet with a composite
structure obtained in the Example;
FIG. 4 is a schematical diagram conceptually showing an expanded
microstructure photograph of a steel sheet with a composite
structure as comparative material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A cold-rolled steel sheet of the present invention is characterized
by a metallographic structure and a chemical component. Description
about metallographic structure specified by the present invention
will be given.
A metallographic structure of a cold-rolled steel sheet of the
present invention under the observation of the optical microscope
is a composite structure composed of a ferrite phase and a second
phase. The second phase has an area rate of 30 to 70%, is combined
approximately in a shape of a network, and is characterized by
approximately uniform distribution of fine ferrite grains in a
structure of a network-like second phase. In more detail, it is
characterized in that a circle-equivalent average ferrite grain
size in the composite structure is not more than 10 .mu.m, and that
a circle-equivalent diameter of a ferrite grain aggregate
continuously existing in an area surrounded by the second phase
combined approximately in a shape of a network is not more than 3
times of the average ferrite grain size.
The second phase is a hard low-temperature transformation product
formed in annealing and cooling process after hot-rolling and
cold-rolling of steel materials. Although the second phase may
partially include bainite, it has tempered martensite or martensite
as a principal component (preferably not less than 50% as area
ratio, and more preferably not less than 80%), and existence of the
hard second phase enables guarantee of high elongation and
high-strength. Although less than 30 area % of an area ratio of the
second phase gives superior elongation, it gives insufficient
strength and insufficient stretch flange formability. An excessive
amount of the second phase exceeding 70 area % causes shortage of
the area ratio of soft ferrite phase, reduces an elongation
percentage, and disables guarantee of a balance of elongation and
stretch flange formability on a level desired in the present
invention. In order to increase both of elongation and stretch
flange formability while securing high-strength, preferable area
ratios of the second phase are not less than 35% and not more than
60%.
Based on the premise of satisfying requirements for the area ratios
of the second phase, an important feature in metallographic
structure enabling differentiation between conventional steel
sheets with a composite structure and cold-rolled steel sheet of
the present invention is that the second phase is precipitated in a
shape of fine, uniform and dense approximate network, and that fine
ferrite grains are finely dispersed almost uniformly, as a small
number of aggregate, in the second phase precipitated in the shape
of the network. Specifically, as example shows in FIG. 1 (a
micrograph in which a ferrite area ratio is 45%) in the
after-mentioned Example, a cold-rolled steel sheet of the present
invention is characterized in that a circle-equivalent average
grain diameter of a ferrite constituting a composite metal
structure is not more than 10 .mu.m, and that a circle-equivalent
diameter of a ferrite grain aggregate, continuously existing in an
area (that is, each network) surrounded by the second phase
(principally tempered martensite or martensite) precipitated
approximately in a shape of a network, is not more than 3 times of
an average ferrite grain size.
For example, FIG. 3 gives more concrete illustration of specificity
of a metallographic structure of a steel sheet with a composite
structure concerning the present invention, and an enlarged drawing
showing schematically photograph substituted for drawing of FIG. 1
mentioned above. In FIG. 3, M shows a second phase precipitated in
a shape of network, F1 and F1 show each ferrite grain, and each of
ferrite grains F1 and F1 is fine (not more than 10 .mu.m of
circle-equivalent average grain diameter), and moreover a network
of a second phase M itself precipitated approximately in a shape of
a network is relatively thin, and the network is also fine. As a
result, a number of ferrite grains F1 and F1 in a ferrite aggregate
existing in the network is very small (In an illustrated example,
the ferrite grains F1 and F1 divided by martensite formed in a
shape of a network mostly form a single crystal grain. Besides,
although two or more ferrite grains F1 may be combined by a very
fine bridge in a cross-sectional structure photograph, grains
combined by such a very thin bridge are regarded as being divided
by the bridge part in the present invention.), and a
circle-equivalent diameter of the ferrite aggregate continuously
existing in an area surrounded by the second phase is controlled by
at most not more than 3 times of the circle-equivalent average
ferrite grain size.
Incidentally, also in steel sheets with a composite structure
mentioned as the conventional technology, observed is precipitation
having an area ratio of a second phase of 40 area % and yet showing
a shape of notionally coarse network. And examples having a
metallographic structure of aggregate of ferrite grains currently
dispersed in a coarse network may exist. For example, a photograph
substituted for drawing of FIG. 2 illustrates the metallographic
structure of the steel sheet with the composite structure produced
with conventional methods. This example is a cold-rolled steel
sheet having a ferrite area ratio in a whole structure of 30%, and
an area ratio of a second phase (martensite) of 70%. As is clear
from FIG. 2, in conventional materials, relatively coarser ferrite
aggregates are distributed among relatively coarser aggregates of a
second phase (martensite) as compared with the present invention.
However, as compared with the FIG. 1 showing the metallographic
structure of the steel sheet of the present invention, the network
structure of the second phase is very coarse, and the second phase
is dispersed as coarse aggregates. In addition, although the
ferrite area ratio is smaller as compared with the sample of the
FIG. 1, only a few ferrite are dotted surrounded by the second
phase in a shape of an island, resulting in continuously existing
ferrite grains.
FIG. 4 is an enlarged drawing schematically showing the
metallographic structure to illustrate the specificity of a
metallographic structure of the conventional steel sheets with a
composite structure. In FIG. 4, M shows a second phase precipitated
in a shape of a network, F1 and F1 show each of ferrite grains, and
F shows ferrite aggregates including ferrite grains existing
continuously. As is clear when FIG. 4 is compared with FIG. 3
(material of the present invention), networks of martensite M is
very coarse, and many of them exist as a big mass. Moreover, each
of ferrite grains F1 and F1 divided by the martensite M is
relatively coarse, and at the same time plural them are combined
together (in FIG. 4, B's are combining sites) to form the ferrite
aggregate F. As a result, a circle-equivalent average grain
diameter of the aggregate F is not less than 3 times of a
circle-equivalent average grain diameter of each of ferrite grains
F1 and F1.
FIGS. 2 and 4 illustrate typical metallographic structures of steel
sheets with a composite structure produced with conventional
methods. Not only in this example, but in conventional steel sheets
with a composite structure when aiming at coexistence of elongation
and stretch flange formability, it has been confirmed that
especially a steel sheet with a composite structure having a
ferrite area ratio exceeding 30% gives circle-equivalent diameters
of a region with a small number of ferrites combined, and of a
ferrite aggregate exceeding 3 times of circle-equivalent average
ferrite grain size, which gives inferior density.
In addition, as is described in detail later, it is confirmed that:
inclusion of Nb, Ti, V, etc. having structure refining effect, as a
steel component, enables refining of the circle-equivalent average
grain diameter of ferrite grains; inclusion of Mo or Cr also
enables, by the structure refining effect, the circle-equivalent
average grain diameter of the ferrite grains to be controlled to
not more than 10 .mu.m; and such refining of ferrite grains can be
attained by the conventional technology as described above.
However, even if a condition of the average grain diameter of
ferrite grains is satisfied, it will be shown clearly in the
after-mentioned Example that a balance of elongation/stretch flange
formability of a level desired by the present invention may not be
obtained, when a circle-equivalent average diameter of a ferrite
aggregate exceeds 3 times of a circle-equivalent average ferrite
grain size.
Therefore, the present invention has indispensable requirements
that, based on a premise of satisfying requirements for component
compositions described later, the metallographic structure has a
composite structure of a ferrite+a second phase; the second phase
is combined approximately in a shape of a network with an area
ratio of not less than 30% and not more than 70%; a
circle-equivalent average ferrite grain size is not more than 10
.mu.m; and a circle-equivalent diameter of a ferrite aggregate
continuously existing in an area surrounded by the second phase is
not more than 3 times of the circle-equivalent average ferrite
grain size.
A more preferable area ratio of the second phase is not less than
40% and not more than 60%. A more preferable circle-equivalent
average ferrite grain size is not more than 7 .mu.m, and although a
minimum value is not especially limited, approximately 2 .mu.m is
considered to be a minimum in consideration of actual operation.
Besides, a preferable circle-equivalent diameter of a ferrite
aggregate is not more than 2 times of a circle-equivalent average
ferrite grain size.
In determination of the metallographic structure, a metallographic
structure is exposed by processing a surface of a section in a
rolling direction of each sample steel sheet with Nital liquid.
Then five places of approximately 80 .mu.m.times.60 .mu.m of area
of sheet thickness of 1/4 were observed by SEM images with
magnification of 1000 times to determine an area ratio of a ferrite
and a second phase, a circle-equivalent average ferrite grain size,
and a circle-equivalent diameter of ferrite aggregate by image
analysis.
Hereinafter, description will be given for chemical compositions of
a steel sheet of the present invention. Hereafter, all units of the
chemical composition are based on mass %.
C: 0.05 to 0.13%
C is an essential element to improve strength. C increases
hardenability and forms hard martensite etc. by low-temperature
transformation, and is indispensable for securing high-strength
essential as a structural material etc. A hardness of the second
phase depends on an amount of C in a composite structure having a
ferrite as a parent phase, and composed of this parent phase and a
second phase (mainly tempered martensite or martensite) as in the
present invention. Therefore, in order to harden the second phase
and to increase stretch flange formability while securing strength,
a content of C is very important and not less then 0.05% of
inclusion is indispensable. Less than 0.05% of content gives
unsatisfactory hardness to the second phase, and it not only
provides insufficient strength as whole but it provides inadequate
stretch flange formability. More preferable C content is not less
than 0.08%. However, since C content exceeding 0.13% excessively
hardens the second phase and reduces elongation and stretch flange
formability, it should be controlled not more than the quantity.
More preferable C content is not more than 0.10%.
Si: 0.5 to 2.5%
Si is useful also as a solid-solution-strengthening element, it has
a function for increasing strength, especially without degrading
stretch flange formability, and in addition it is an element useful
for expanding transformation-temperature range and enabling easy
control of metallographic structure. In order to effectively
exhibit such a function, not less than 0.5% of content is
necessary, and preferably not less than 1.0% of content. However,
since excessive Si content has an adverse effect on stretch flange
formability and elongation, and degrades chemical conversion
treatability etc., and therefore desirably the content is
controlled not more than 2.5%, and more preferably not more than
2.0%.
Mn: 0.5 to 3.5%.
Mn is an element for promoting hardening like the C. In order to
form sufficient amount of hard second phase through low-temperature
transformation by hardening after annealing, not less than 0.5% of
content is necessary, preferably not less than 1.0%, and more
preferably not less than 1.5%. However, since an excessive amount
of Mn makes area ratios of the second phase increase rapidly and it
also markedly reduces elongation and stretch flange formability, an
amount should be controlled not more than 3.5%. More preferable Mn
content is not more than 3.0%, and more preferably not more than
2.5%.
Mo: 0.05 to 0.6% and/or Cr: 0.05 to 1.0%
These Mo and Cr are important additional elements in the present
invention. Although theoretical reasons are not yet clarified
enough, experimental results show functions of increasing hardness
of a ferrite parent phase and of concurrently controlling hardness
of the second phase, and while the elements exhibit very important
function in order to reduce a ratio of hardness and a hardness
difference between the ferrite phase and the second phase, and to
increase elongation and stretch flange formability, they contribute
also to improvement in hardness as a whole steel. In order to
effectively exhibit such a function, inclusion of not less than
0.05% of Cr and not less than 0.05% of Mo is indispensable.
Inclusion of not less than 0.10% of Mo and not less than 0.20% of
Cr is preferred. These may be included independently, and two sorts
may be added in combination. However, since excessive inclusion of
those elements reduces homogeneity of structure, deteriorates
stretch flange formability, preferably Mo is controlled not more
than 0.6%, and Cr is controlled not more than 1.0%. In case of
compound addition of Mo and Cr, in order to avoid occurrence of the
fault a total amount is preferably controlled not more than 1.2%.
Besides, it is confirmed that such an improvement effect of
elongation and stretch flange formability by Mo and Cr is markedly
promoted conjointly with refining, uniformity, and denseness of the
composite metal structure.
In addition to the components, a steel sheet of the present
invention may include following components.
At least One Kind Selected from a Group Composed of Ti: 0.005 to
0.05%, Nb: 0.005 to 0.05%, and V: 0.005 to 0.2%
These Ti, Nb, and V have precipitation-accelerating and
structure-refining effect, and especially refine a ferrite grain
size, and contribute to improvement in elongation. In addition,
they have function of improving strength and stretch flange
formability by structure refining as whole. The function is
effectively exhibited by inclusion of not less than the lower limit
of each component, but since each of contents exceeding each of
maximum values reduces elongation and adversely affects
elongation/stretch flange formability balance, careful attention
must be paid for the amounts thereof. Principal elements in a steel
sheet with a composite structure concerning the present invention
are described above, and remainder is substantially Fe, but
following elements may be included in a range that does not impair
operational advantage distinctive to the present invention
mentioned above.
Al: not more than 0.10%
Al functions effectively as a deoxidizer, and it is an element
effective also for reducing a function that prevents refining of
ferrite grain caused by dissolved N by fixing N, as AlN, possibly
mixed into an ingot steel. However, since an excessive quantity
makes a ferrite parent phase coarser and has adverse influence on
stretch flange formability, it should be controlled not more than
0.10%.
S: not more than 0.005%
S is a harmful element generally having adverse influence on
workability and strength of steel. Since it has significant adverse
influence in stretch flange formability also in the present
invention, it should be controlled not more than 0.005%.
N: not more than 0.01%
It is thought that N is effective because N reacts with the Ti, Nb,
V, Al, etc., to form nitride, and contributes to refining of a
ferrite phase. However, when much amount of N content increases an
amount of dissolved N, since it will have significant adverse
influence in elongation or stretch flange formability it should be
controlled not more than 0.01%.
P: not more than 0.03%
P is considered to be a harmful element generally degrading
weldability of steel, and it is desirable to be controlled not more
than 0.03% also in the present invention.
The Al, S, N, P, etc. are elements mixed unavoidably in ingot
stages, and are preferably decreased as much as possible,
respectively, based on the reasons mentioned above. Besides these
elements, suitable amount of addition of, for example, Cu, Ni, Co,
W, Zr, B, Ca, REM, etc. enables effective use of function of these
elements in a range not giving adverse influence on operational
advantage aimed by the present invention.
As mentioned above, a cold-rolled steel sheet of the present
invention has a high strength not less than 780 MPa and exhibits a
balance of elongation/stretch flange formability exceeding
conventional materials by satisfying a specific metallographic
structure and a specific chemical composition. Those detailed
values show not less than 14% of an elongation (El), and not less
than 50% of a stretch flange formability (.lamda.), and both of
them are physical properties exceeding those of conventional
materials. Incidentally, in conventional technology mentioned
above, as will be clarified also in the after-mentioned Examples,
materials showing not less than 14% of an elongation (El) show a
stretch flange formability (.lamda.) less than 50%, and materials
showing not less than 50% of a stretch flange formability (.lamda.)
show less than 14% of an elongation (El) by common examining
methods. Thus, a steel sheet with a composite structure being able
to satisfy both of the elongation and stretch flange formability
cannot be obtained.
In a steel sheet with a composite structure of the present
invention having the distinctive balance of elongation/stretch
flange formability, a characteristic thereof appears directly also
in a ratio (HvII/Hv.alpha.) between an average hardness (HvII) of
the second phase and an average hardness (Hv.alpha.) of a ferrite
parent phase, and the steel sheet is characterized in that the
ratio shows a low value of not more than 3.0, and preferably not
more than 2.0. That is, although it is confirmed, also in
conventional steel sheets with a composite structure, that a small
ratio (HvII/Hv.alpha.) mentioned above is preferable in order to
increase a balance of elongation/stretch flange formability, the
ratio exceeds 3.0 also in examples having small ratios, and
examples having the ratio not more than 3.0 are not known.
Therefore, a steel sheet with a composite structure of the present
invention also may be recognized to have characteristic physical
property showing a low ratio (HvII/Hv.alpha.) not more than
3.0.
As mentioned above, a steel sheet with a composite structure of the
present invention exhibits high-strength, and superior
elongation/stretch flange formability balance by possessing a
proper component composition and a characteristic metallographic
structure, and methods for manufacturing the steel sheet are not
especially limited. Preferable manufacturing conditions for
obtaining the proper steel sheet with a composite structure will,
hereinafter, be illustrated, on condition that a steel satisfying
requirement for the chemical composition is used as a material.
That is, a steel satisfying the requirements for the component is
smelted, a slab is obtained by continuous casting or ingot making,
and, subsequently it is hot-rolled. In hot-rolling, after a finish
temperature of finish rolling is set not less than Ar3 point, and
appropriately cooled, a rolled steel is coiled in a temperature
range of 450 to 700.degree. C. After hot-rolling, the rolled sheet
is pickled and then cold-rolled. A cold-rolling rate is preferably
set not less then about 30%.
Recrystallizing annealing and cooling performed after cold-working,
and furthermore a processing condition of subsequent overaging are
important processes in order to obtain a steel sheet with a
composite structure by formation of a second phase structure as a
low-temperature transformation product. After recrystallizing
annealing at a temperature of not less than Ac1 point, a sheet is
cooled at a rate of 10 to 30.degree. C./s, and then it is hardened
by quenching at a rate of not less than 100.degree. C./s from a
temperature range of 700.degree. C. to 600.degree. C., and
furthermore, is overaged in a temperature range of 180.degree. C.
to 450.degree. C.
In order to avoid remaining of processing structure of the hot
rolled steel sheet, a finishing temperature of hot-rolling is set
at not less than Ar3 point, and thus a composite structure
comprising a low-temperature transformation product and a ferrite
may be obtained by coiling in a temperature range of 450 to
650.degree. C. The low-temperature transformation product means a
martensite and a bainite. In the present invention, the second
phase preferably has a martensite as a main constituent, and more
preferably not less than approximately 70% of the second phase is
of a martensite. In order for the second phase to have a martensite
as a main constituent, a cooling rate following the annealing
process is set high as mentioned later.
A cold-rolling rate is set not less than 30% in order to promote
recrystallization, an austenite phase is formed in the annealing
process by performing the recrystallizing annealing (a soaking
temperature) at a temperature of not less than Ac1 point, and then
a partial ratio is set as 30 to 70% by subsequent cooling.
The austenite phase is transformed into a low-temperature
transformation product comprising the martensite (or tempered
martensite and bainite) by following cooling. In order to prevent
precipitation of a perlite or increase of a ferrite phase, the
cooling rate is set at least not less than 10.degree. C./s, and
preferably not less than 30.degree. C./s. An ultra high-speed
cooling as water quenching etc. is also preferred.
After quenching, aging (annealing) treatment is performed for
hardness adjustment of the low-temperature transformation product.
An excessively low aging temperature fails to diffuse carbon, and
excessively high aging temperature conversely causes too much
softening, resulting in insufficient strength. Therefore, an aging
treatment is desirably carried out in a range of 180 to 400 degrees
C. for about 1 to 10 minutes.
For example, adoption of the above manufacturing conditions may
satisfy requirements for a metallographic structure mentioned above
in combination with a steel component mentioned above, and
simultaneously may provide a steel sheet with a composite structure
having a high-strength, and a well-balanced elongation/stretch
flange formability in a high level. In order to realize a structure
characterized in the present invention, a cooling rate after
hot-rolling finishing and soaking temperature conditions of an
annealing process in manufacturing conditions are important
requirements.
EXAMPLE
Although the present invention will, hereinafter, be described more
in detail with reference to Examples, the present invention is not
at all limited by the following Examples. Of course, the present
invention may suitably be carried out in a range that may suit the
above and the after-mentioned spirit, and each of the modification
is included by a technical scope of the present invention.
Test sample steels (unit in Table is mass %) having component
compositions indicated in following Table 1 were smelted, slabs
were obtained with a conventional method, and then the slabs
obtained were hot-rolled on conditions shown in Table 2 to obtain
2-mm-thick hot-rolled steel sheets. After pickling, the steel
sheets were cold-rolled into a thickness of 1.2 mm, and they were
annealed on conditions shown in the Table.
Sections in 5 areas (about 80 .mu.m.times.60 .mu.m) of sheet
thickness of 1/4 in a rolling direction of obtained steel sheet
were observed as images with 1000 times of magnification by SEM.
Area ratios of a ferrite and the second phase, circle-equivalent
average grain diameters of ferrite and circle-equivalent diameters
of ferrite aggregate were obtained using image analysis. Here,
regions continued out of a view of ferrite aggregates were excluded
from analysis. Ferrite grains and the second phases having average
grain diameters were measured for Vickers hardness according to JIS
Z 2244.
Tension test was carried out according to JIS Z 2241, and JIS No. 5
test pieces of the steel sheet were measured for strength (TS) and
total elongation (El), and 100 mm square steel sheets were measured
for hole expanding ratios (.lamda.) according to Japan Iron and
Steel Federation specification JFST1001. Table 3 shows results.
Each the second phase structure of sample steel sheets obtained in
this experiment was substantially only of martensite, and others
were of ferrite as a main phase.
TABLE-US-00001 TABLE 1 (mass %) Kind of steel C Si Mn P S Al Cr Mo
Additional elements 1 0.125 1.44 1.50 0.011 0.0010 0.05 0.48 0.13 2
0.064 1.12 2.03 0.007 0.0015 0.03 0.46 0.18 3 0.091 0.78 1.64 0.012
0.0018 0.05 0.32 0.23 4 0.115 1.68 1.76 0.010 0.0015 0.04 0.26 0.28
5 0.098 1.25 1.07 0.006 0.0009 0.03 0.43 0.16 6 0.071 1.56 2.40
0.007 0.0017 0.05 0.78 0.21 7 0.111 0.84 1.09 0.011 0.0020 0.05
0.26 0.03 8 0.101 0.94 1.30 0.016 0.0011 0.04 0.87 0.02 9 0.087
1.33 1.30 0.018 0.0021 0.04 0.02 0.12 10 0.111 1.07 2.34 0.008
0.0006 0.04 0.04 0.56 11 0.098 1.47 1.41 0.008 0.0008 0.05 0.72
0.44 12 0.094 1.03 2.22 0.014 0.0020 0.03 0.68 0.34 Nb: 0.021 13
0.094 0.81 1.32 0.011 0.0021 0.03 0.41 0.12 Ti: 0.043 14 0.073 1.48
1.56 0.010 0.0017 0.04 0.63 0.29 V: 0.12 15 0.043 1.17 1.01 0.009
0.0022 0.05 0.55 0.18 16 0.145 1.36 2.43 0.010 0.0005 0.03 0.63
0.26 17 0.097 0.41 1.40 0.020 0.0012 0.04 0.79 0.20 18 0.077 2.62
1.85 0.016 0.0015 0.04 0.44 0.09 19 0.088 1.31 0.32 0.018 0.0016
0.03 0.38 0.19 20 0.081 1.00 3.65 0.013 0.0021 0.04 0.45 0.17 21
0.107 0.98 1.75 0.012 0.0024 0.05 0.03 0.02 22 0.112 1.13 2.29
0.018 0.0013 0.05 1.13 0.18 23 0.072 1.40 1.07 0.010 0.0008 0.03
0.10 0.87
TABLE-US-00002 TABLE 2 Hot-rolling conditions Annealing Cooling Air
condition Forced cooling Finishing Primary termination cooling
Secondary Coiling up Soaking starting Overaging Referential Kind
temperature cooling rate temperature period cooling rate
temperature temperature temperature temperature numeral of steel
(.degree. C.) (.degree. C./s) (.degree. C.) (s) (.degree. C./s)
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) 1 1 900 35
690 8 30 480 900 660 270 2 2 890 60 690 8 35 520 880 670 230 3 3
880 55 670 11 25 520 830 680 240 4 4 900 50 660 12 35 490 860 650
280 5 5 880 40 670 7 35 510 860 650 270 6 6 890 50 680 6 40 530 860
640 240 7 7 880 45 660 13 30 560 890 660 270 8 8 890 45 660 6 40
550 890 650 290 9 9 910 55 680 10 25 500 850 650 260 10 10 870 50
690 12 20 520 830 640 250 11 11 880 40 690 10 25 500 850 640 280 12
12 890 30 680 7 35 530 880 660 220 13 13 860 50 660 8 20 530 900
680 290 14 14 890 30 690 13 40 540 840 680 270 15 15 900 50 650 10
45 520 890 630 320 16 16 890 35 680 7 30 550 880 700 230 17 17 880
60 650 7 45 500 890 680 300 18 18 870 60 660 10 35 490 880 670 290
19 19 880 30 660 11 45 490 850 620 270 20 20 890 55 680 13 30 550
850 680 280 21 21 900 45 670 12 10 630 840 650 310 22 22 890 35 680
8 35 540 880 680 270 23 23 900 40 870 5 50 530 840 620 290
TABLE-US-00003 TABLE 3 Microstructure Circle-equivalent Hardness
Mechanical property Referential VII d.alpha. diameter of .alpha.
HvII Hv.alpha. YS TS El .lamda. numeral Kind of steel (%) (.mu.m)
aggregate (.mu.m) (Hv) (Hv) HvII/Hv.alpha. (MPa) (MPa) (%) (%) 1 1
47 3.0 4.7 413 194 2.1 743 992 16.3 64 2 2 61 4.2 5.9 364 199 1.8
842 1027 15.3 82 3 3 49 6.4 9.3 382 184 2.1 784 985 15.7 65 4 4 51
7.8 10.9 380 185 2.1 760 995 16.4 65 5 5 33 2.0 3.5 407 171 2.4 566
791 20.4 79 6 6 58 2.9 4.3 433 198 2.2 925 1112 14.3 85 7 7 33 7.6
9.8 422 179 2.4 584 804 19.4 63 8 8 52 2.6 4.9 396 183 2.2 765 1006
15.7 62 9 9 34 6.7 9.4 405 178 2.3 574 797 20.1 87 10 10 64 2.7 3.7
369 208 1.8 926 1093 14.1 71 11 11 54 5.4 8.4 374 210 1.8 779 1024
16.0 56 12 12 63 4.1 6.0 436 192 2.3 966 1143 14.3 78 13 13 47 2.4
3.6 402 183 2.2 773 1018 15.3 56 14 14 53 5.2 8.9 356 195 1.8 753
984 16.6 63 15 15 22 10.4 53.4 389 166 2.3 547 762 21.1 23 16 16 70
11.6 42.5 491 182 2.7 1079 1276 9.6 38 17 17 53 3.6 11.3 438 132
3.3 815 1015 15.1 34 18 18 33 8.2 35.1 366 221 1.7 777 1076 16.0 32
19 19 24 6.6 17.3 449 138 3.3 426 669 24.5 39 20 20 72 4.7 8.9 390
229 1.7 1199 1215 10.4 62 21 21 39 12.7 44.5 419 176 2.4 664 854
18.1 32 22 22 68 5.8 22.6 386 218 1.8 1122 1297 9.2 36 23 23 53 2.9
12.1 372 229 1.6 811 1105 14.8 27
In Tables 1 to 3, referential numerals 1 to 14 represent Examples
satisfying all requirements for regulation of the present
invention. They have proper chemical compositions, and hot-rolled
conditions, and subsequent cooling conditions and annealing
conditions are suitable to provide preferable metallographic
structures, and therefore they can show tensile strengths exceeding
780 MPa in a high level, and they also show high values of
elongations and stretch flange formability.
On the other hand, since referential numerals 15 to 23 lack either
of requirements of the present invention, they have problems, as
follows, in some performance aimed by the present invention.
Since referential numeral 15 has an insufficient C content, it has
a small partial ratio of a second phase. And ferrite grains thereof
are excessively combined together and therefore a low strength and
inferior hole expanding property are exhibited. Although a
referential numeral 16 has many C contents and it has comparatively
few ferrite phases, many of ferrite grains are combined together.
Since a referential numeral 17 has a small Si content and it has a
large hardness ratio of a ferrite and a second phase, poor hole
expanding property is exhibited. A referential numeral 18 has an
excessive Si content, and combination of ferrite grains advances
and inferior hole expanding property is exhibited.
A referential numeral 19 has an inadequate Mn content and an
inadequate second phase partial ratio, exhibits large hardness
ratio of ferrite and the second phase, and furthermore exhibits
unsatisfactory strength and hole expanding property. A referential
numeral 20 has an excessive Mn content, and second phase partial
ratio, and exhibits poor ductility. A referential numeral 21 has
inadequate Cr and Mo content, and therefore has coarse ferrite
grains, and furthermore since it has many ferrite grains combined
together, it exhibits inferior hole expanding property. Referential
numerals 22 and 23 have excessive Cr and Mo content respectively,
and simultaneously since they have many ferrite grains combined
together, they show inferior hole expanding property.
FIG. 1 is a SEM photograph showing a microstructure of a steel
sheet with a composite structure (referential numeral 2) in Example
of the present invention, wherein the structure consists of a
second phase (martensite) combined together in a shape of a thin
network, and a ferrite phase divided with the network and finely
dispersed (main phase). A crystal grain diameter of each ferrite is
fine, and simultaneously there is a little average number of
ferrite grains in ferrite aggregates divided with network composed
of martensite, and therefore the ferrites are finely dispersed as a
whole. On the other hand,
FIG. 2 is a SEM photograph showing a microstructure of a
referential numeral 16 as comparison material, wherein a second
phase (martensite) coarsely solidified as compared with Example
material of FIG. 1 is roughly dispersed, and a state may be
confirmed where large ferrite aggregates having many ferrite grains
combined together therebetween are roughly dispersed.
That is, comparison of FIG. 1 and FIG. 2 clarifies that the Example
material of FIG. 1 has a very dense and wholly uniform
microstructure, but on the other hand the comparative material of
FIG. 2 has a coarse and wholly uneven microstructure.
The foregoing invention has been described in terms of preferred
embodiments. However, those skilled, in the art will recognize that
many variations of such embodiments exist. Such variations are
intended to be within the scope of the present invention and the
appended claims.
* * * * *